(EAM) Charging by Friction Lesson
Charging by Friction
Suppose that a rubber balloon is rubbed with a wool sweater. During the rubbing process, the atoms of the rubber are forced into close proximity with the atoms of the wool. The electron clouds of the two types of atoms are pressed together and are brought closer to the nuclei of the other atoms. The protons in the atoms of one material begin to interact with the electrons present on the other material. Of course, the atoms of one material - in this case, the atoms of rubber - are more serious about their claim for electrons. As such, the atoms of rubber begin to take electrons from the atoms of wool. When the rubbing has ceased, the two objects have become charged.
The frictional charging process results in a transfer of electrons between the two objects that are rubbed together. Rubber has a much greater attraction for electrons than wool. As a result, the atoms of rubber pull electrons from the atoms of wool, leaving both objects with an imbalance of charge. The rubber balloon has an excess of electrons and the wool has a shortage of electrons. Having an excess of electrons, the rubber balloon is charged negatively. Similarly, the shortage of electrons on the wool leaves it with a positive charge. The two objects have become charged with opposite types of charges as a result of the transfer of electrons from the least electron-loving material to the most electron-loving material.
Charging by Conduction
Charging by conduction is a method used to charge a neutral object when in direct contact with another charged object. For example, the transfer of electrons from a Van de Graaff generator to the woman seen in the picture below is charging by conduction. Conduction occurs because there is direct contact between materials that differ in their ability to give up or accept electrons. A Van de Graaff generator produces a negative charge on its dome, so it tends to give up electrons. Human hands are positively charged, so they tend to accept electrons. Therefore, electrons flow from the dome to the woman's hand when they are in contact.
You don't need a Van de Graaff generator for conduction to take place. It may occur when you walk across a wool carpet in rubber-soled shoes. Wool tends to give up electrons and rubber tends to accept them. Therefore, the carpet transfers electrons to your shoes each time you put down your foot. The transfer of electrons results in you becoming negatively charged and the carpet becoming positively charged. The important thing to remember about charging by conduction is that there must be direct contact in order for the charging to occur.
Charging by Induction
Induction charging is a method used to charge an object without actually touching the object to any other charged object. When you bring an object with static electricity charges on its surface near another object, those charges will induce opposite charges to come to the surface. This works best with conductors, which allow charged particles to move around fairly freely, but also works with nonconductors to a lesser degree.
The electroscope—a device used to detect static electricity—uses this principle to charge its detecting parts. The electroscope consists of a glass container with a metal rod inside which has 2 thin pieces of gold foil attached. The other end of the metal rod has a metal plate attached to it outside the glass container.
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Neutral Electroscope |
The electroscope detects charge in the following way: A charged object, like the positively charged rod in the picture above, is brought close to (but not touching) the neutral metal plate of the electroscope. This causes the negative charge in the gold foil, metal rod, and metal plate, to be attracted to the positive rod. Because the metal (gold is a metal too!) is a conductor, the charge can move freely from the foil up the metal rod and onto the metal plate. There is now more negative charge on the plate and more positive charge on the gold foil leaves. This is called inducing a charge on the metal plate. It is important to remember that the electroscope is still neutral (the total positive and negative charges are the same), the charges have just been induced to move to different parts of the instrument! The induced positive charge on the gold leaves forces them apart since like charges repel! This is how we can tell that the rod is charged. If the rod is now moved away from the metal plate, the charge in the electroscope will spread itself out evenly again and the leaves will fall down because there will no longer be an induced charge on them.
Review static electricity using the activity below.
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